A conventional computing device (e.g., smart phone, tablet computer, etc.) may include a system on chip (SOC), which has a processor and other operational circuits. The SOC may receive its power from a battery, and thus conventional designs may balance SOC performance and power usage to deliver a desirable experience to the user while requiring as little battery charging as practicable.
Some conventional SOC designs include multiple power domains receiving power from one or more power supplies. Power multiplexing may be used in some conventional systems to reduce power consumption during normal operation of a computing device. For instance, a power multiplexor may include a multiplexor that receives at its inputs multiple (e.g., two) power inputs and has a power output, and the power multiplexor selects between the power inputs. And a given SOC design may include a multitude of different power multiplexors to provide power to different processing units in the SOC.
One way in which some conventional systems may use power multiplexing to save power is to enable a power collapse of some parts of a processing core (using a first power multiplexor and a first power domain) while providing power to parts of the processing core that store state values (using a second power multiplexor and a second power domain). Both the first and second power multiplexors may select between the first and second power domains. Another way in which some conventional systems may use power multiplexing is to switch from a first power supply to a second power supply to power a central processing unit (CPU) memory and then adjusting the second power supply to overdrive the CPU memory. This technique may save power by allowing the SOC to selectively raise a voltage at some components while not raising the voltage at other components.
Thus, a power multiplexor (or power mux) may be used to switch cores between two or more power supplies, depending on operating mode. A conventional power multiplexor may include a mixed signal design, employing analog components (e.g., a VDD comparator) to detect the higher of two supplies and an analog voltage generator that generates the higher of the two (or multiple) voltages that are being switched to an internal supply. The analog circuitry, such as the comparator and voltage generator, may be complex and use an undesirably large amount of circuit space. Accordingly, there is a need in the art for power multiplexor designs that omit analog comparators and voltage generators.
Such conventional power multiplexors may use head switches having a single transistor each. The conventional power multiplexor may operate under the assumption that only one side is active at any given time, allowing switching between power domains. However, if a transistor used in one of the head switches is not fully turned off, it may allow leakage between the power domains, which may be undesirable. Accordingly, there is a need in the art for a more reliable power multiplexor.